1. Field of the Invention
The invention relates in general to protective relays, and more specifically to protective relays for protecting high-voltage transmission line sections in a three-phase electrical power system.
2. Description of the Prior Art
Pilot protection for high-voltage electrical transmission lines identifies faults on the protected line section by using communication channels for relays to compare system conditions at the terminals or ends of the line section. For economic reasons, power line carrier or microwave pilot channels are used for long transmission lines, and pilot-wire relaying is used for protecting short transmission line sections, i.e., usually about ten miles, or less. Pilot-wire relaying is very important, as in short high-voltage lines, discrimination is difficult with distance type-relays, making pilot relaying the only practical method of discrimination not based on time delay.
Pilot-wire relays require a continuous metallic circuit between the terminals of the protected line section. A composite sequence filter at each terminal converts the three-phase currents into a single-phase voltage, and the single-phase voltage is applied to the pilot-wire pair for comparison with the other single phase voltage. The protective relay at each terminal is an electromechanical relay which includes an operating coil and a restraint coil. The operating and restraint coils are connected in the pilot-wire system such that with normal through current in the protected line section, the polarities of the two single-phase voltage developed by the composite sequence filters cause more current to flow in the restraint coils than in the operating coils. An internal fault, i.e., a fault in the protected section, causes one of the single-phase voltages to shift its phase angle and the instantaneous polarities are then such that more current flows through the operating coils than through the restraint coils, causing the relays to simultaneously trip their associated circuit breakers to clear the protected transmission line section. This tripping characteristic to set forth in the second and fourth quadrants of the normal operating characteristic of a pilot-wire relay system.
The electromechanical pilot-wire relay also has the unique feature of being able to operate the relays sequentially even when the instantaneous polarities of the single-phase voltages do not indicate an internal fault, i.e., the first and third quadrants of the tripping characteristic, when the current at one terminal is much higher than the current at the other terminal, indicating a loss of current due to a fault in the transmission line section to be protected. Thus, both relays may be tripped on an internal fault on the protected line section even when one terminal has a very weak source, or temporarily, no source at all. In the electromechanical relay, this feature is inherently achieved due to the normal linear operation converting to a non-linear operation due to saturation, for currents exceeding about 200% of nominal pick-up.
The electromechanical pilot-wire relay system securely, reliably and economically provides high speed simultaneous clearing of both line terminals of a protected transmission line section due to a fault in the protected section. Continuous, end-to-end metallic circuits, for use in pilot-wire relaying, which are typically leased from a telephone company, are becoming increasingly more difficult to obtain, or even retain. Thus, it would be desirable to provide a new and improved pilot relaying system which has the desirable characteristics of discriminatory in-phase and out-of-phase tripping, and the economic attractiveness of the pilot-wire relaying system, but which does not require that the communication link be a continuous metallic circuit.